Patent Document 1 discloses a conventional method of manufacturing a large- or medium-sized wheel disk including steps of: (a) ironing a blank of a flat plate to a first-stage product of a cup-like configuration having a cylindrical rising portion; and (b) diametrically enlarging the cylindrical rising portion of the first-stage product to a conical rising portion thereby forming the first-stage product to a second-stage product.
However, there are the following problems with the conventional method of manufacturing a large- or medium-sized wheel disk:
More particularly, in Patent Document 1, at the ironing step, an entirety of a portion of the blank corresponding to the rising portion of the first-stage product is ironed. However, in Patent Document 1, there is no teaching or suggestion about taking the next, second step of diametrically enlarging the rising portion into consideration when conducting the first step of ironing. Therefore, there is the possibility that cracks may be caused in the product material when diametrically enlarging the first-stage product to the second-stage product, because after having ironed the product material, the length of the ironed portion of the product material is significantly decreased.